Publish and broadcast announcements to users

[pcrespov]

@eofli or any other member of the app-team, would like to broadcast public announcements (e.g. student-contest, offers, new services, etc ) to all users at the landing page.

Tasks

Beta Give feedback

1. ✨ Frontend: Student competition announcement osparc-simcore#4404
   a:frontend changelog:✨new-feature +2
   
2. ✨ Announcements entrypoint at the web-api (⚠️ devops) osparc-simcore#4487
   a:webserver +1
   
3. ✨ Frontend: Announcements osparc-simcore#4489
   a:frontend changelog:✨new-feature +2
   
4. Entrypoint to publish announcements restricted to role (e.g. tester, owner, admin)

[pcrespov]

@eofli if you have a chance, please fill up more details in the description.thx